The constantly increasing voltages in electrical transmission networks require greater sparkover widths between the live power supply and apparatus parts and the earth. This requires great insulator lengths. If active parts of the apparatus must be housed inside the insulator, the diameters of the insulators are also large. Against the influences of the atmosphere and other environmental influences, which reduce the insulating strength, projecting annular protective screens must be provided for the above mentioned high voltage in order to increase the creepage path. In general, these large insulators represent rather complicated structures.
The production of large insulators requires considerable engineering effort which manifests itself in high costs. Depending on the insulating material and the technology used, large construction gages, molds, baking and hardening furnaces as well are required. With increasing insulator size, the waste and shipping risk similarly increase.
There has been no lack of attempts in the past to optimize the manufacture of insulators both technically and economically. It has been known for a long time how to make large insulators from several burnt partial insulators. Clips are used as connecting means which are arranged as press elements inside the insulator. Recently, cementing has been added as a new technique. Thus, a large insulator cemented from several partial insulators is known from the German publication "STEMAG Nachrichten" no. 40 of April 1967, particularly p. 1076. The form of the adhesion surfaces, however, particularly of the plane surfaces, did not meet the requirements of simple manufacture, strength of joint, protection against environmental influences and esthetic design, which these surfaces and the finished insulator must have.